1. Field of the Invention
The present invention relates to a granular charging agent used in electrically charging a material and a method of electrically charging a material using the same, and more particularly, a method of uniformly charging an electrophotographic photoconductor for use in a variety of image forming apparatus such as a printer and a copying machine using the above-mentioned granular charging agent.
2. Discussion of Background
Since the invention of an electrophotographic process by C. F. Carlson as disclosed in U.S. Pat. No. 2,297,691, various improvements and developments have been made based on the aforementioned Carlson process. According to the electrophotographic image formation method based on the Carlson process, which is now widely employed, image formation is basically carried out in such a manner that the surface of a photoconductor is uniformly charged to a predetermined polarity and the photoconductor thus charged is selectively exposed to the original light images to form electrostatic latent images on the photoconductor. Then, the electrostatic latent images are developed with a developer, so that visible toner images can be obtained on the photoconductor. The visible toner images are then transferred to a sheet of an image-receiving member and fixed thereon.
To uniformly charge the surface of the photoconductor, two kinds of charging methods are conventionally employed. One is a non-contact charging method, such as corona charging; and the other is a contact charging method, such as roller charging, brush charging, charging by use of particles (hereinafter referred to as the particle charging) and triboelectric charging.
In particular, the corona charging is most widely used as the method of electrically charging the photoconductor. However, the corona charging has the drawbacks that the charging level becomes unstable over an extended period of time, ozone generated in the charging process is bad for the health of human body, and a product generated in the charging process is deposited on the surface of the photoconductor.
Of various kinds of contact charging methods, on he other hand, the roller charging method is put to practical use. Although ozone is not generated during the charging process by the roller charging, the roller charging has the problems that there is a risk of the photoconductor being broken by abnormal discharge, it is difficult to uniformly charge the photoconductor, and the environmental stability is poor.
The brush charging is carried out using a flat brush with electroconductive fine fibers which is fixed or freely rotated. However, the fine fibers of the brush are laid flat or stained during the repeated operation, which causes the problem that the charge-imparting capability of the charging brush is decreased.
The particle charging method, which employs electroconductive particles, is carried out in such a manner that electric charge is injected into a material, such as a photoconductor, through a magnetic brush composed of magnetic particles. More specifically, according to the particle charging method, the photoconductor is conventionally charged using the magnetic particles with a volume resistivity of about 10.sup.3 to 10.sup.7 .OMEGA..multidot.cm, with a bias voltage of as high as 1 kV or more being applied thereto. For example, a charging bias voltage of 2,000 V is applied to magnetic charging particles with a volume resistivity of 10.sup.6 .OMEGA..multidot.cm in the charging process as disclosed in Japanese Laid-Open Patent Application 61-57958. In addition, as disclosed in Japanese Laid-Open Patent Application 59-133569, when magnetic particles with an electrical conductivity of 10.sup.-7 V/cm are used for the charging agent, the photoconductor with a surface potential of 500 V can be obtained in the charging process by the application of a voltage of 600 V to the charging magnetic particles.
However, uniform charging cannot be achieved by the conventional particle charging method. Furthermore, since the granular charging agent (hereinafter also referred to as charging particles) is transferred to the surface of the photoconductor and deposited thereon, it is necessary to provide a blade for removing the charging particles deposited on the photoconductor therefrom downstream from the charging position in the rotational direction of the photoconductor.
In addition, when the electroconductivity of the granular charging agent is increased to improve the capability of charging the photoconductor, the electric charge is concentratedly injected in minute flaws provided in the surface of the photoconductor. This may damage the photoconductor. As previously mentioned, it is very difficult to control the charging capability of the granular charging agent adequately in the conventional particle charging method.